Project Summary Atrial fibrillation (AF) is the most common cardiac arrhythmia in the United States and is associated with poor quality of life, an increased risk of thromboembolic events (namely embolic stroke), and increased risk of mortality. Observational studies have demonstrated multiple risk factors for the onset of AF, yet more emphasis is needed on modifiable risk factors. Lifestyle habits such as alcohol consumption and smoking are potentially intervenable behaviors that could drastically alter the risk of AF. Moreover, the effect of secondhand smoke and risk of AF has not yet been fully studied. Our preliminary data suggests that exposure to smoking during early development increases risk of incident AF. The effect of smoking as a potentially modifiable risk factor requires further elucidation in order to develop effective strategies that can meaningfully impact AF prevention. Therefore, we propose to further characterize the relationship between tobacco smoke exposure and risk of AF. The overall hypothesis of our project is that there is an association between tobacco smoke exposure and AF risk whether that exposure is indirect (secondhand smoke) or direct. We will test this hypothesis in the following aims: Aim 1 will assess secondhand smoke exposure in early development and risk of incident AF later in adulthood; Aim 2 will query abrupt changes in secondhand smoke exposure during adulthood and AF hospitalizations on a population level; Aim 3 will investigate the direct effect of inhaled tobacco and acute onset of AF episodes as measured by a continuous cardiac monitor in patients with known paroxysmal AF.